The present invention relates to a method and apparatus for transmitting signals in a serial form from a control device to a diagnostic device of a motor vehicle by means of a diagnostic data lead interconnected between the control device and the diagnostic device.
Extremely complicated electronic switching devices have been always utilized in the electrical part of a motor vehicle. These devices have been used, for example for defining the time point of the ignition or the start time point and the duration of the fuel injection. For example, an ignition pulse can be released in dependence upon the parameters of the motor vehicle.
In order to ensure a reliable operation of the control device, which is normally operated with the aid of microprocessors, it has been necessary to examine a routine operation of the control device by means of diagnostic devices provided on the vehicle in spaced relationship with each other. For this purpose the diagnostic devices are connected to the control device of the motor vehicle, thereby the function of the control device has been examined. The transmission of signals generated by the control device has been obtained in the serial form in a diagnostic data lead. In order to cause the control device to transmit data to the diagnostic device stimulus signals from the diagnostic device have been delivered to the control device, or certain arrangements in the control device have been necessary. This procedure has been actually troublesome because the data transmission must have been induced by an external handle. On the one hand, there has been a possibility that in the case of malfunction false data could be released, which could be connected with a defect of the control device, and on the other hand, an actuation of a switch, for example in the control device could lead to the fact that the switch, after being inserted into the motor vehicle, would remain in the diagnostic position, so that the control device would not reliably operate in that inserted position.